1. Field of the Invention
This invention relates in general to dispersive interdigital transducers for acoustic waves which have split finger arrangements.
2. Description of the Prior Art
Prior art structures such as shown, for example, in transducers using acoustic waves such as described in the article Pro. IEEE, Ultrasonics Symposium (1974), pages 224 through 227 and the same magazine of 1981 on pages 181 through 185 discloses for example electric filter resonator arrangements that operate with acoustic waves which travel in a substrate in at least one surface-proximate region of the substrate. Such acoustic waves are known as Rayleigh waves, Bleustein waves, Love waves, SSBW waves, SABB waves and these are referred to below generally as surface waves although strictly speaking only the two former wave types are surface waves.
Such arrangements with acoustic waves comprise piezoelectric properties at least in the region of the interdigital transducers employed as input transducers and/or as output transducers. An interdigital transducers which is employed uses finger electrodes which are arranged parallel to each other on the surface of the substrate and these are usually distributed as two comb-shaped structures which are formed by the fingers and respective bus bars. The comb structures interdigitally engage each other and the finger electrodes are usually strip-shaped metallization codings on the substrate surface.
For the acoustic wave running in the surface region of the substrate, such finger electrodes also act as reflectors which is undesirable in interdigital transducers. There have been many proposals as to how such undesired reflections occurring at the finger electrodes of an interdigital transducer can be reduced or eliminated. A first such measure in known split finger devices of interdigital transducers is to divide each individual finger into two fingers which are parallel next to one another and which are electrically connected to the same polarity and which are both connected to the same bus bar. In the simple interdigital transducer the center to center spacing from one finger electrode to the neighboring oppositely polarized following finger electrode usually is .lambda./2 where .lambda. is the wavelength of the acoustic wave. This center to center spacing is also the same as the center to center spacing from one piezoelectric excitation center between two oppositely polarized finger electrodes to the neighboring excitation center. This definition also applies to an interdigital transducer used as a reception transducer. Accordingly, with the center to center spacing of .lambda./2 it is standard to make a finger electrode with dimensions of .lambda./4 and the clearance between two neighboring finger electrodes as .lambda./4.